A Zeroeth Law is a law that is added to another set of laws at a later date.

It is usually so important that the other laws cannot function without it, yet so obvious that nobody thought it needed stating.

The Zeroeth Law of thermodynamics: If objects B and C are both in thermal equilibrium with object A, then they are in thermal equilibrium with each other.

Isaac Asimov's Zeroth Law of Robotics: A robot may not harm humanity, or through inaction allow humanity to come to harm.

Newton's Zeroeth Law of Motion: If objects A and B are moving relative to object C at velocities v1 and v2, then they are moving relative to each other at velocity v3 which is the difference between v1 and v2. (Note: This law is now known not to be true, especially near the speed of light. That's where Einstein comes in)

The Zeroeth Commandment: Thou Shalt believe all of this and never question what thou art told.